557 research outputs found
Local Control of Reactive Power by Distributed Photovoltaic Generators
High penetration levels of distributed photovoltaic (PV) generation on an
electrical distribution circuit may severely degrade power quality due to
voltage sags and swells caused by rapidly varying PV generation during cloud
transients coupled with the slow response of existing utility compensation and
regulation equipment. Although not permitted under current standards for
interconnection of distributed generation, fast-reacting, VAR-capable PV
inverters may provide the necessary reactive power injection or consumption to
maintain voltage regulation under difficult transient conditions. As side
benefit, the control of reactive power injection at each PV inverter provides
an opportunity and a new tool for distribution utilities to optimize the
performance of distribution circuits, e.g. by minimizing thermal losses. We
suggest a local control scheme that dispatches reactive power from each PV
inverter based on local instantaneous measurements of the real and reactive
components of the consumed power and the real power generated by the PVs. Using
one adjustable parameter per circuit, we balance the requirements on power
quality and desire to minimize thermal losses. Numerical analysis of two
exemplary systems, with comparable total PV generation albeit a different
spatial distribution, show how to adjust the optimization parameter depending
on the goal. Overall, this local scheme shows excellent performance; it's
capable of guaranteeing acceptable power quality and achieving significant
saving in thermal losses in various situations even when the renewable
generation in excess of the circuit own load, i.e. feeding power back to the
higher-level system.Comment: 6 pages, 5 figures, submitted to IEEE SmartGridComm 201
High-harmonic generation: taking control of polarization
The ability to control the polarization of short-wavelength radiation generated by high-harmonic generation is useful not only for applications but also for testing conservation laws in physics
On possible implications of gluon number fluctuations in DIS data
We study the effect of gluon number fluctuations (Pomeron loops) on deep
inelastic scattering (DIS) in the fixed coupling case. We find that the
description of the DIS data is improved once gluon number fluctuations are
included. Also the values of the parameters, like the saturation exponent and
the diffussion coefficient, turn out reasonable and agree with values obtained
from numerical simulations of toy models which take into account fluctuations.
This outcome seems to indicate the evidence of geometric scaling violations,
and a possible implication of gluon number fluctuations, in the DIS data.
However, we cannot exclude the possibility that the scaling violations may also
come from the diffusion part of the solution to the BK-equation, instead of
gluon number fluctuations.Comment: 9 pages, 2 figures; references added, minor changes, matches
published versio
Using Higher Moments of Fluctuations and their Ratios in the Search for the QCD Critical Point
The QCD critical point can be found in heavy ion collision experiments via
the non-monotonic behavior of many fluctuation observables as a function of the
collision energy. The event-by-event fluctuations of various particle
multiplicities are enhanced in those collisions that freeze out near the
critical point. Higher, non-Gaussian, moments of the event-by-event
distributions of such observables are particularly sensitive to critical
fluctuations, since their magnitude depends on the critical correlation length
to a high power. We present quantitative estimates of the contribution of
critical fluctuations to the third and fourth moments of the pion, proton and
net proton multiplicities, as well as estimates of various measures of
pion-proton correlations, all as a function of the same five non-universal
parameters, one of which is the correlation length that parametrizes proximity
to the critical point. We show how to use nontrivial but parameter independent
ratios among these more than a dozen fluctuation observables to discover the
critical point. We also construct ratios that, if the critical point is found,
can be used to overconstrain the values of the non-universal parameters.Comment: 16 pages, 5 figures. Version to appear in PRD. Footnote and reference
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Enhancement and suppression of tunneling by controlling symmetries of a potential barrier
We present a class of 2D systems which shows a counterintuitive property that
contradicts a semi classical intuition: A 2D quantum particle "prefers"
tunneling through a barrier rather than traveling above it. Viewing the one
particle 2D system as the system of two 1D particles, it is demonstrated that
this effect occurs due to a specific symmetry of the barrier that forces
excitations of the interparticle degree of freedom that, in turn, leads to the
appearance of an effective potential barrier even though there is no "real"
barrier. This phenomenon cannot exist in 1D.Comment: 10 pages and 7 figure
Coherent-incoherent transition in the sub-Ohmic spin-boson model
We study the spin-boson model with a sub-Ohmic bath using a variational
method. The transition from coherent dynamics to incoherent tunneling is found
to be abrupt as a function of the coupling strength and to exist for
any power , where the bath coupling is described by . We find non-monotonic temperature dependence of the
two-level gap and a re-entrance regime close to the transition due
to non-adiabatic low-frequency bath modes. Differences between thermodynamic
and dynamic conditions for the transition as well as the limitations of the
simplified bath description are discussed.Comment: 12 pages, 4 figure
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